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Fat and dry in the Sahel; the Reed Warbler’s double life

As we made our way to our ringing site, walking over dried, cracked earth, it was hard to believe that one of our target species made its living here. For a couple of weeks for the last few winters I have been part of a small research team led by, James Vafidis and Rob Thomas, that has headed to Senegal to look at how migratory birds utilise habitats during the non-breeding season.

Dry scrub; not the most inviting looking of habitats

We tend to think of the reed warbler Acrocephalus scirpaceus as a wetland-specialist. But this diminutive bird leads a double life; when it arrives in the Sahel region of Africa, it becomes a habitat-generalist. While some of the wintering population head to the wetlands of Senegal, others opt to make a living in the dry-scrubland. Seeing a reed warbler flitting amongst the acacia scrub, as the windblown dust coats your binoculars, is a sight I’ve yet to become accustomed too.

The reed warbler’s insect prey is much more scare in dry-scrub than in the wetland. We found that reed warblers in dry scrub fend off starvation by building up fat reserves. In contrast, those warblers inhabiting wetlands take advantage of the high insect abundance there and remain lean throughout winter. Wetland-specialists, such as Eurasian sedge A. schoenobaenus warbler also maintain reduced body reserves.

A net in the wet; insects are abundant in the reed swamps of Senegal

But why do reed warblers lead this double life? Why not just head to the wetland like the sedge warblers? It could be that they are competitively excluded from these habitats. The wetlands are replete with resident, insectivorous species such as the African reed warbler, with which seasonal visitors must contend against for resources. Then there are the other migrants; sedge warblers which undertake migration in almost a single bound are likely to arrive earlier in the Sahel, as are individuals from more southerly populations. Could it be that reed warblers or certain populations of the species, out flanked by sedge warblers in habitats near to saturation had no choice but to move into the dry scrub?

This exclusion has had its benefits as this ability to exploit low quality habitats gives the reed warbler a competitive edge. Over winter survival in the sedge warbler is inextricably linked to rainfall in the Sahel; in drought years, the sedge warbler’s habitat resource is diminished. The reed warbler’s habitat flexibility, enabling it to exploit low quality habitats, enables the species to survive the winter means that regardless of conditions in the Sahel. This strategy could be the reason why the reed warbler’s UK breeding population is on the rise, while the sedge warbler is in decline.

The results of these forays, the first study to attempt to investigate mass regulation strategies among warblers in these two wintering habitats in the Sahel, have recently been published in the journal PlosOne. Like all research it has opened up more questions than it answered. Perhaps just one more trip to Senegal is needed…